perf(client): cache parsed chunk/batch origin coordinates

[RZDESIGN]

Summary

Chunk.chunkIdToOriginCoordinate() and Chunk.batchIdToBatchOrigin() convert string IDs like "32,0,-64" into { x, y, z } objects. They're called in hot per-frame loops:

Caller	Frequency
ChunkMeshManager.applyBatchViewDistance()	Every batch, every frame
ChunkManager batch sorting	Every batch rebuild
LightLevelManager / SkyDistanceVolumeManager	On volume events
ChunkWorker	On every chunk build
Chunk.getChunkIdsInBatch() / chunkIdToBatchId()	Various

Each uncached call performs:

1. split(',') → allocates an Array<string>
2. .map(Number) → allocates a closure + a new Array<number>
3. { x, y, z } → allocates a result object

That's 5 allocations per call. With ~200 active batches, applyBatchViewDistance() alone creates ~1,000 throwaway objects per frame — pure GC pressure for a deterministic mapping that never changes.

Fix

Add a module-level Map<string, Vector3Like> cache for each method. The first call for a given ID parses and caches; all subsequent calls return the cached object. Since the mapping from string ID to coordinates is deterministic and all callers are read-only, this is completely safe.

Changes

File	Change
client/src/chunks/Chunk.ts	Added chunkOriginCache and batchOriginCache maps; wrapped both methods with cache-first lookup

Risk Assessment

• Correctness: The mapping "x,y,z" → { x, y, z } is pure and deterministic — caching it cannot produce incorrect results.
• Mutation safety: All callers read .x, .y, .z without mutation. If a future caller were to mutate the returned object, it would corrupt the cache — but this would also be a bug in the caller, not in the cache.
• Memory: The cache grows with unique chunk/batch IDs. A large world with 10,000 chunks would store ~10,000 entries (trivial — each entry is a string key + 3-number object, roughly 100 bytes).
• No eviction needed: Chunk/batch IDs are stable for the lifetime of the world. The caches are module-scoped and reset naturally on page reload.

Test Plan

• Load a large world and move around — verify chunks render and cull correctly (view distance, frustum)
• Check that light level volumes still work near light-emitting blocks
• Monitor GC pauses / minor GC frequency — expect a measurable reduction in worlds with many batches